Multiple sclerosis (MS) is a chronic human autoimmune disease of the central nervous system (CNS) that affects 600,000 Americans and 2.5 million individuals worldwide. MS is most often diagnosed between the ages of 20 and 50 and is second only to trauma in causing neurological disability in young adults. The disease usually starts between 20 to 40 years of age and there are two major forms. Relapsing-remitting MS (RR-MS) is the most frequent form (85%-90%) and affects women about twice as often as men. Most RR-MS patients later develop the second major form known as secondary progressive MS (SP-MS). About 10%-15% of patients show a steady progression following disease onset with the absence of relapses, termed primary progressive PP-MS. (Sospedra, et al., Annu Rev Immunol 23, 683 [2005]). MS is a highly heterogeneous disease where every patient differs in clinical presentation and response to treatments.
Although the exact cause of MS is unknown, pathologically there is inflammation-induced destruction of the myelin sheath that surrounds axons in the brain and spinal cord leading to decreased nerve conduction. Clinically, the loss of myelin leads to a variety of neurological symptoms and, in some patients, major disability. In most patients with relapsing-remitting disease, inflammation-induced demyelination is spontaneously repaired by oligodendrocytes, the cells in the brain that produce and maintain myelin. Acute inflammation and chronic demyelination eventually lead to destruction of oligodendrocytes and axonal loss. The secondary progressive phase of MS is characterized by neurodegeneration and treatment-resistant functional deterioration.
Current treatment options for MS are immunomodulatory and immunosuppressive therapies that are mostly effective during the inflammation-mediated relapsing-remitting phase of MS. These therapies are only partially effective in slowing down the progressive phase of MS, which may be largely neurodegenerative. There is an urgent need for therapies that can stop or reverse the progression of MS through strategies involving neural repair and regeneration.
Stem cell therapies hold much promise for regenerative medicine. Stem cells have the potential to develop into many different cell types in the body. Stem cells can theoretically divide without limit to replenish cells in need of repair. There are different types of stem cells with varying ranges of commitment options. Embryonic stem cells hold great potential for regenerative medicine, however, they have a number of disadvantages including the possibility of transplant rejection and possible teratoma formation if the cells are not properly differentiated prior to transplantation. Adult stem cells such as neural stem cells (NSC) and oligodendrocyte precursor cells (OPC) have a more restricted developmental potential than embryonic stem cells and generally differentiate along their lineage of origin. While adult neural stem cells also represent a promising treatment option for neurodegenerative disorders, there are a number of disadvantages, including difficulty of isolation, limited expansion capability, and immune rejection of transplanted donor cells.
Bone marrow-derived mesenchymal stem cells (MSCs) are another type of adult stem cell that differentiates into non-hematopoietic tissues including osteoblasts, adipocytes, chondrocytes, and myoblasts (Ferrari, et. al., Science 279:1528-1530 [1998]; Pittenger, et. al., Science 287:143-147 [1999]; Prockop, et. al., Science 276:71-74 [1997]). The use of bone marrow-derived stem cells has many therapeutic advantages. Bone marrow is an easily accessible and autologous source of stem cells, thus eliminating the risk of rejection. Since mesenchymal stem cells have enormous ex vivo expansion capability, it is possible to expand a small population of cells into enough cells for clinical application.
MSCs have a number of remarkable in vitro characteristics, making them a very attractive candidate for neurodegenerative and immunological disorders.
MSCs exhibit differentiation plasticity, meaning that they are capable of differentiating along lineages other than their tissue of origin (Jiang, et. al., Nature 418:41-49 [2002]; Woodbury, et. al., J. Neurosci Res 69:908-917 [2002]). MSCs are capable of forming cells with neuronal and glial phenotypes in vitro (Black et. al., Blood Cells Mol Dis 27: 632-636 [2001]; Deng et. al., Biochem Biophys Res Commun 282: 148-152 [2001]; Hermann, et. al., J. Cell Sci 117:4411-4422 [2004]; Sanchez-Ramos, et. al., Exp. Neurol 164: 247-256 [2000]; Suzuki, et. al., Biochem Biophys Res Commun 322: 918-922 [2004]; Woobury, et. al., J. Neurosci Res 61: 364-370 [2000]). MSCs incubated in the presence of growth factors basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) displayed a neural stem cell morphology with increased expression of neural stem cell markers (Hermann, et. al., J Cell Sci 117: 4411-4422 [2004]). These studies raise the possibility that MSCs may be capable of cell replacement in the damaged brain and spinal cord.
MSCs have an irnmunoregulatory function. It has been demonstrated that MSCs can suppress T, B, NK, and dendritic cell activation and proliferation (Uccelli, et. el., Expert Opin Biol Ther 6: 17-22 [2006]). These anti-inflammatory properties of MSCs suggest a possible clinical application for immune-mediated disease.
MSCs can promote the genesis of neurons and oligodendrocytes from neural stem cells (Bai, et. al., Neurochem Res 32: 353-362 [2007]; Rivera, et. al. Stem Cell 24: 2209-2219 [2006]). These recent studies show that MSCs secrete trophic factors that influence neural stem cell progeny, which may have clinical implications in enhancing recovery after a wide range of CNS injuries.
The findings that MSCs can generate neural stem cell-like cells that express neural stem cell markers (Hermann, et. al., J Cell Sci 117: 4411-4422 [2004]) suggest that MSC-derived neural precursors may be a more potent source for therapeutic use in the CNS. MSCs cultured in neural stem cell-specific media (serum free media containing 20 ng/ml of both EGF and bFGF) exhibited neurosphere morphology with an increase in neural stem cell marker genes (Nestin), glial genes (GFAP, MBP) and neuronal genes (Map2, Neurofilament, Tyrosine hydroxylase, voltage dependent K+ channels) (Hermann, et. al., J Cell Sci 117: 4411-4422 [2004]; Hermann, et. al., J Neurosci Res 83: 1502-1514 [2006]; Mareschi, et. al., Exp Hematol 34: 1563-1572 [2006]). In comparing different culture conditions to convert MSCs to neural precursors, one study found that MSCs cultured in Neural Progenitor Maintenance Media (NPMM, Lonza) acquired the morphological characteristics, neural markers, and electrophysiological properties suggestive of neural differentiation (Mareschi, et. al., Exp Hematol 34: 1563-1572 [2006]).
Use of Mesenchymal Stem Cells in Neurological Diseases
A number of clinical trials have analyzed the safety and therapeutic benefit of MSCs. Several hundred patients have been infused with allogeneic HLA-matched MSCs in the context of hematopoietic stem-cell transplant for malignancy or inborn metabolic disease (Lazarus, et. al., Biol Blood Marrow Transplant 11: 389-398 [2005]; Le Blanc, et. al., Biol Blood Marrow Transplant 11: 321-334 [2005]). Severe graft vs. host disease (GvHD) was reversed upon infusion of donor-derived MSCs (Le Blanc et. al., Lancet 363: 1439-1441 [2004]). In these studies, infusion of approximately 1-2 million cells/kg MSCs is well tolerated with no side effects. Thus, initial clinical trial data suggests that MSC-based therapies hold much promise as immune modulators, and additional Phase I/II trials studying the effects of the role of MSCs in the treatment of GvHD are ongoing (Giordano, et. al., J Cell Physiol 211: 27-35 [2007]). Furthermore, clinical trials examining MSCs in osteogenesis imperfecta (Horwitz, et. al., Proc Natl Acad Sci USA 99: 8932-8937 [2002]; Le Blanc, et. al., Transplantation 79: 1607-1614 [2005]), myocardial infarct (Chen, et. al., Am J Cardiol 94: 92-95 [2004]; Katritsis et. al., Catherter Cardiovas Intery 65: 321-329 [2005]), and stroke (Bang, et. al., Ann Neurol 57: 874-882 [2005]) have shown safety and efficacy for the treatment of these diseases as well.
In a recent clinical trial in Italy (Mazzini, et. al., Amyotroph Lateral Scler Other Motor Neuron Disord 4: 158-161 [2003]; Mazzini, et. al., Neurol Res 28: 523-526 [2006]; Mazzini, et. al., Neurol Sci [2007]), autologous bone marrow-derived MSCs were transplanted directly into the spinal cord of nine patients with amyotrophic lateral sclerosis (ALS), a degenerative motor neuron disease. Intraspinal transplantation of an average of 32 million autologous MSCs was safe and well tolerated by ALS patients, within a follow-up period of 4 years. Minor adverse events were intercostal pain irradiation and leg sensory dysesthesia, which disappeared after 6 weeks. In 5 of the patients receiving autologous MSC transplantation, there was a significant slowing of the linear decline in the ALS-functional rating scale (Mazzini, et. al., Neurol Res 28: 523-526 [2006]). Given the progressive nature of this disease, these findings suggest a clinical benefit of MSC transplantation and warrant further trials. The encouraging clinical results for autologous MSC transplantation in ALS and the safety and tolerability observed in the 4-year follow-up suggest that a similar approach could be taken to treat other neurodegenerative disorders such as MS.
Use of Mesenchymal Stem Cells in MS
Based on the preclinical data demonstrating peripheral immunosuppression in EAE, a current phase I/IIA research study is taking place at the University of Cambridge in Cambridge, England. The study aims to investigate the safety of intravenous administration of autologous MSCs in patients with multiple sclerosis. There are 20 patients enrolled in the study, and the dosage will be 2 million cells/kg.
In a recent pilot study conducted in Iran, human autologous MSCs were injected intrathecally in ten primary progressive and secondary progressive MS patients (Mohyeddin, et. al., Iran J Immunol 4: 50-57 2007]). Researchers injected an average of 8.73 million cells. The highest dose was 13.2 million cells and the lowest dose was 2.5 million cells. Patients were followed for an average of nineteen months after treatment, with monthly follow-up exams and an MRI 12 months after intrathecal injection of autologous MSCs. Patients were evaluated for changes in EDSS score, changes in the number and size of lesions on an MRI, and subjective improvement. Researchers found that intrathecal autologous MSC transplantation was safe and well tolerated with side effects related to the intrathecal injection procedure (e.g. headache). Two patients contracted iatrogenic meningitis and both patients were treated successfully with antibiotics. They also noted that the therapy was associated with some improvement. One patient experienced a decrease in EDSS score. Four patients showed improvement in daily functions with no change in EDSS score. Five patients experienced an increase in EDSS score, and all five reported subjective improvement within three months of treatment. Researchers concluded that autologous intrathecal MSC injection is a safe and promising treatment for MS patients.
The use of MSC-derived neural precursors has not hereto before been reported for administration to humans.